1. Field of the Invention
This invention relates to a method for the heat treatment of a ZnSe crystal substrate to dope it with Al as a donor impurity, a ZnSe crystal substrate prepared by this heat treatment and a light-emitting devices using the ZnSe crystal substrate.
2. Description of the Related Art
ZnSe-based light-emitting devices have been noticed with keen interest in particular, as light-emitting devices of LED and LD in a region of blue, green or blue green. These light-emitting devices can also be formed on a GaAs substrate, but from the standpoint of improving the property of such a device, it is, of course, desirable to prepare a ZnSe-based thin film by homoepitaxial growth on a ZnSe substrate. For the purpose of simplifying the device structure and improving the device property, an electrically conductive, low resistance substrate is indispensable. However, a ZnSe bulk single crystal ordinarily prepared by a PVT (Physical Vapor Transport) method or GG (Grain Growth) method, etc. is undoped and highly resistant.
Up to the present time, as a method for rendering a ZnSe single crystal low-resistant, it has been proposed to subject the ZnSe single crystal to a heat treatment in a Znxe2x80x94Al melt in an atmosphere of Zn, thus obtaining a low resistance ZnSe single crystal (J. Phys. D: Appl. Phys. Vol. 9, 1976, p. 799-810, G. Jones et al.). According to this method, Al can be diffused in the ZnSe crystal to act as a donor impurity, while simultaneously, the Zn vacancy concentration can be lowered to increase the activation efficiency of Al and to increase the n-type carrier concentration, thus resulting in a desired specific resistance.
In this heat treatment method, however, there arise some problems that the ZnSe single crystal cannot be prevented from adhesion of the Znxe2x80x94Al solid during cooling, the dislocation density of the ZnSe single crystal is increased and its crystalline property is markedly deteriorated because of stress due to difference in thermal expansion coefficients between ZnSe and the Znxe2x80x94Al solid. In order to solve this problem, there has been proposed a method comprising forming an Al thin film on a surface of ZnSe single crystal and then subjecting them to a heat treatment in Zn atmosphere in a closed vessel (Japanese Patent No. 02839027). According to this method, marked deterioration of the crystalinity has been suppressed.
In a ZnSe substrate in which Al is diffused with a high concentration, the absorption edge shifts towards the longer wavelength side to cause such a phenomenon that blue light and blue-green light are absorbed and longer wavelength lights of from yellow to red are re-emitted (which longer wavelength light emission is generally called SA emission). Utilizing this phenomenon, white LED has been developed. This LED is capable of emitting white light by mixing blue green light from the activation layer and yellow SA emission from the substrate.
In the case of preparing a monochromatic LED of blue or blue-green, however, there is a problem such that a pure monochromatic light is hard to obtain because of mixing of this SA emission.
It is an object of the present invention to provide a method for the heat treatment, whereby the above described problems can be solved and a ZnSe crystal substrate having an electrical conductivity and good crystallinity as well as a low SA emission intensity can be prepared with good re-producibility.
It is another object of the present invention to provide a ZnSe crystal substrate prepared by the above described method for the heat treatment.
It is a further object of the present invention to provide a light-emission device using the ZnSe crystal substrate.
These objects can be attained by a method for the heat treatment of a ZnSe crystal substrate by diffusing Al through the ZnSe crystal substrate, comprising previously forming an Al film on the substrate, first subjecting the substrate to a heat treatment in a Se atmosphere and then subjecting to a heat treatment in a Zn atmosphere.